Sub-ambient refrigerating cycle

ABSTRACT

According to one embodiment of the invention, a method for cooling heat-generating structure disposed in an environment having an ambient pressure includes providing a fluid refrigerant and reducing a pressure of the refrigerant to a first sub-ambient pressure at which the refrigerant has a boiling temperature less than a temperature of the heat-generating structure. The method also includes bringing the refrigerant at the first sub-ambient pressure into thermal communication with the heat-generating structure, so that the refrigerant boils and vaporizes to thereby absorb heat from the heat-generating structure. The method further includes increasing a pressure of the vaporized refrigerant above the first sub-ambient pressure to a second sub-ambient pressure.

TECHNICAL FIELD OF THE INVENTION

This invention relates in general to cooling techniques and, moreparticularly to a method and apparatus for cooling a system thatgenerates a substantial amount of heat.

BACKGROUND OF THE INVENTION

Some types of electronic circuits use relatively little power, andproduce little heat. Circuits of this type can usually be cooledsatisfactorily through a passive approach, such as convection cooling.In contrast, there are other circuits that consume large amounts ofpower, and produce large amounts of heat. One example is the circuitryused in a phased array antenna system.

Electronic circuits and other structures that generate relatively largeamounts of heat may be cooled through well know refrigeration systems.However, suitable refrigeration units are large, hefty and consume manykilowatts of power in order to provide adequate cooling. One reason forthis is that typical refrigerants in these types of systems tend to havea low change of phase energy, requiring a large flow rate for removal ofheat. The combination of high flow rates and high pressure causes highcompressor work. Thus, although refrigeration units of the above typehave been generally adequate for their intended purposes, they have notbeen satisfactory in all respects.

In this regard the size, weight and power consumption characteristics ofthe known refrigeration systems are all significantly larger thandesirable for certain apparatuses generating large amounts of heat.Given that there is an industry trend towards even greater powerconsumption in heat dissipation in certain types of electronics, such asphased array antenna systems, continued use of conventionalrefrigeration-based cooling systems would continue to result in evengreater size, weight and power consumption, which is undesirable.

SUMMARY OF THE INVENTION

According to one embodiment of the invention, a method for coolingheat-generating structure disposed in an environment having an ambientpressure includes providing a fluid refrigerant and reducing a pressureof the refrigerant to a first sub-ambient pressure at which therefrigerant has a boiling temperature less than a temperature of theheat-generating structure. The method also includes bringing therefrigerant at the first sub-ambient pressure into thermal communicationwith the heat-generating structure, so that the refrigerant boils andvaporizes to thereby absorb heat from the heat-generating structure. Themethod further includes increasing a pressure of the vaporizedrefrigerant above the first sub-ambient pressure to a second sub-ambientpressure.

Embodiments of the invention may provide numerous technical advantages.Some embodiments may benefit from some, none, or all of the followingadvantages. According to one embodiment, an efficient, lightweightrefrigeration system is provided that has a large cooling capacity, butrequires less power than conventional refrigeration systems. Inparticular, cooling may occur in an ambient environment having atemperature greater than the heat-generating structure that is beingcooled. In some embodiments water is used as a refrigerant and providesa high degree of heat transfer, enabling an efficient heat transfersystem. In addition, water does not result in harmful effects to theenvironment associated with many common refrigerants. Such a system mayalso allow for the use of a smaller heat exchanger than would otherwisebe required.

Other advantages may be readily apparent to one skilled in the art.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete understanding of the invention and its advantages willbe apparent from the detailed description taken in conjunction with theaccompanying drawings in which:

FIG. 1 is a block diagram of an apparatus according to the teachings ofthe invention; and

FIG. 2 is a graph illustrating a thermal cooling cycle according to theteachings of the invention.

DETAILED DESCRIPTION OF EXAMPLE EMBODIMENTS OF THE INVENTION

Example embodiments of the present invention and its advantages are bestunderstood by referring to FIGS. 1 and 2 of the drawings, like numeralsbeing used for like and corresponding parts of the various drawings.

FIG. 1 is a block diagram of a system 10 for cooling according to theteachings of the invention. As illustrated, system 10 includesheat-generating structure 12, which in this example is electroniccircuitry and, in particular, is a phased array antenna. Althoughelectronic circuitry is used as an example for heat-generating structure12, system 10 may be used to cool any suitable heat-generatingstructure, including use as a home cooling system. In that example, anelectronic cold plate may be in thermal communication with both thephased array antenna and refrigerant within a refrigeration loop 17, asdescribed below. System 10 also has a compressor 14 and a heat exchanger16 included within the refrigeration loop 17.

A refrigerant within cooling loop 17 is maintained at a sub-ambientpressure that is less than the pressure of the ambient environment,represented by reference numeral 19. By maintaining the pressure of therefrigerant in loop 17 at a sub-ambient pressure, refrigerants such aswater, which typically boil at temperatures too high to be used as arefrigerant, may be utilized. The use of water as a refrigerant providesseveral advantages. In particular, the boiling of water provides a highdegree of heat transfer, enabling an efficient heat transfer system. Inaddition, water does not result in harmful effects to the environmentassociated with many common refrigerants. Ethylene glycol may also beadded to water and the mixture used as the refrigerant. Otherrefrigerants may also be used, including conventional ones, depending onthe saturation pressure of the refrigerant and the desired coolingtemperature. In general, the refrigerant may be selected by any standardselection criteria used in the industry.

The pressure of the refrigerant between heat-generating structure 12 andcompressor 14 is maintained approximately at a first sub-ambientpressure. Without the use of compressor 14, a refrigerant at the firstsuch sub-ambient pressure could provide a good cooling system in whichheat-generating structure 12 is cooled by coming into thermalcommunication with the liquid refrigerant, causing the liquidrefrigerant to boil at its saturation temperature and change into itsvapor form. The heat stored in the vapor refrigerant is then transferredby exchanger 16 to the outside environment. However, a problem with sucha system is that the ambient temperature of ambient environment 19 withwhich heat exchanger 16 exchanges heat could not practically be greaterthan the temperature of the heat-generating structure 12. This issatisfactory in some circumstances; however, there are many instances inwhich heat-generating structure 12 is at a temperature that is near orless than the temperature of ambient environment 19.

To address this problem, according to the teachings of the invention,compressor 14 is provided in loop 17 between heat-generating structure12 and heat exchanger 16. Providing such a compressor 14 results inlowering the saturation temperature on the low pressure side ofcompressor 14 and thus the temperature at which heat is exchanged atheat-generating structure 12 can be lowered, such that when heat isexchanged by heat exchanger 16 to the outside environment, it may beexchanged at a temperature that is greater than the temperature ofheat-generating structure 12. However, the pressure on the high pressureside of compressor 14 remains at sub-ambient levels. In contrast toconventional refrigeration systems, compressor 14 does not result in alarge pressure differential, and in many applications provides only afew psi pressure increase. This is often large enough to allow exchangeof heat by heat exchanger 16 to the outside environment at a highertemperature than possible without compressor 14.

As merely one example, the temperature at which heat-generatingstructure 12 is generating heat is 50° C. and the temperature withinloop 17 at heat-generating structure 12 is 1.8 psia. However, thepressure on the high pressure side of compressor 14 is 4.54 psia at atemperature of 70° C. According to one embodiment, loop 17 is filledwith an appropriate amount of refrigerant and loop 17 is evacuated untilthe desired saturation pressure (which is below ambient pressure) isachieved. This could be performed with any suitable structure, includinga vacuum pump (not explicitly shown).

Thus, through the use of a refrigerant that is maintained at asub-ambient pressure in conjunction with the use of a compressor with arelatively small power input, an efficient, refrigeration system may beprovided that has a large cooling capacity. In particular, cooling mayoccur in an ambient environment having a temperature greater than theheat-generating structure that is being cooled.

Such a system may also allow for use of a smaller heat exchanger, suchas heat exchanger 16, than would otherwise be required. Such a systemmay be applied in any suitable context, including military applicationsas well as an alternative to commercial air conditioning systems usinghigh pressure compressors. Further, use of a sub-ambient refrigerationsystem allows use of water as a refrigerant, which has an associatedhigh value of phase change energy and is more environmentally friendlythan conventional refrigerants.

In certain embodiments, a pump 18 may be desired to circulate therefrigerant within loop 17. In particular, use of pump 18 may becombined with orifices 22. Orifices 22 may be provided to allowselective control of cooling of various portions of electronics 12. Asdescribed above, electronics 12 may take the form of a phased arrayantenna in which selective cooling of various portions of antenna systemmay be desired (separate portions not explicitly shown). Such a phasedarray antenna system is described in greater detail in co-pendingapplication entitled “Method and Apparatus for Cooling With Coolant at aSubambient Pressure”, filed Jul. 11, 2002, having a Ser. No. of10/192,891, and an attorney docket number of 004578.1262, which isincorporated herein by reference for all purposes. Such cooling may beeffected by pumping selective amounts of the refrigerant in loop 17through orifices 22 to selected portions of electronics 12. Pump 18boosts the liquid refrigerant pressure for orifice control.

In conjunction with orifices 22, a temperature sensor and feedbacksystem 24 may be provided at the output of electronics 12 to measure thetemperature of the refrigerant flowing through various portions ofelectronics 12. This temperature may be fed back to an orifice flowcontroller 25 to allow modification of the amount of refrigerant flowthrough orifices 22 and therefore to various portions of heat-generatingstructure 12.

After thermal communication with heat-generating structure 12, it islikely that portions of the refrigerant within loop 17 will remain inliquid form. An accumulator 20 is provided, in one embodiment, toaccumulate portions of refrigerant that have not vaporized and tominimize liquid flow to compressor 14, however, it may be desirable tohave some liquid flow past such an accumulator to compressor 14. In anexample in which the refrigerant is a combination of water and ethyleneglycol, accumulator 20 accumulates the ethylene glycol, which is likelyto remain in liquid form. In this embodiment, a second pump 26, whichmay be a relatively small pump with low pressure differential, may beprovided to raise the pressure of the accumulated liquid such that itmay be reintroduced into loop 17 and provided back to heat-generatingstructure 12 for further cooling.

According to another aspect of the invention, the teachings of theinvention recognize that use of a sub-ambient refrigeration system couldresult in leaks of non-condensable ambient air into loop 17. Sub-ambientsystems are counter-intuitive because of the potential for leaks intothe system. However, the teachings of the invention recognize thebenefits of such a system and ways to address such leaks. The teachingsof the invention recognize that such air would tend to be trapped on thehot side of heat exchanger 16 and could result in a lower inner heattransfer coefficient.

To address this problem, in one embodiment, a gas removal system 28 isprovided. Alternatively, gas removal system 28 could be replaced with aport that allows periodic discharge of air with a vacuum pump or othersuitable device on a periodic basis. In the embodiment in which gasremoval system 28 is provided, gas removal system 28 may include a smallvolume but large pressure differential compressor 30 that raises thepressure of the entrapped air above ambient for venting to theenvironment. In another embodiment, this may be combined with providingthe resultant air at an increased pressure along with vapor to heatexchanger 32 to transfer some of the heat in the mixture to the ambientenvironment. This results in condensing some of the vapor to liquidform. The resulting mixture is provided to separator 34. Restrictionvalve 36 allows communication of the condensed liquid back into loop 17from separator 34, while a vent 38 allows venting of the air as well asvapor to the atmosphere. As described above, compressor 30 increases thepressure of the trapped air to above ambient, so that it may be ventedoutside system 10 to the outside environment. Gas removal system 28 maybe operated periodically as needed, or may operate continuously.

FIG. 2 is a graph illustrating a thermal cooling cycle according to theteachings of the invention. Reference to this graph illustratesadvantages of the use of the above-described sub-ambient refrigerationsystem using water as a refrigerant, as opposed to a conventionalrefrigeration system using R22 as a refrigerant. The followingcalculations for the cooling system according to the teachings of theinvention utilizing water has the refrigerant and for a conventionalrefrigeration system using R22 illustrate a fifty percent improvement inthe coefficient of performance of the cooling system of the presentinvention as compared to a conventional system. FIG. 2 also illustratesan alternative thermal cooling cycle utilizing points 3′ and 4′, inwhich liquid is carried over to avoid high temperatures in thecompression process and potentially make the process more efficient.

Water:

-   -   h₁=h₂=125.89 Btu/lb (Ref. P₁=4.54 PSIA, P₂=1.8 PSIA)    -   h₃=1114.5 Btu/lb, v₃=194.1 Ft³/lb    -   s₃=s₄=1.9294 Btu/lb-R, h₄=1180. Btu/lb    -   COP=Useful Heat Removal/Work        Required=(h₃−h₂)/(h₄−h₃)=(1114.5−125.89)/(1180.−1114.5)    -   COP=15.1

R22:

-   -   h₁=h₂=59.2 Btu/lb (Ref. P₁=434.75 PSIA, P₂=281.84 PSIA)    -   h₃=112.45 Btu/lb, s₃=0.20517 Btu/lb-R    -   h₄=117.5 Btu/lb, v₃=0.18573 lb/Ft³    -   COP=(112.45−59.2)/(117.5−112.45)=10.54

Ideal Comparison of Water and R22:

Heat in at 50C, Heat Rejection at 70C, Three Ton Refrigeration (10.55kW) Water R22 COP 15.1 10.5 Work (Hp) 0.937 1.34 PD (Ft3/min) 117.8 2.09Mass (lb/min) 0.607 11.3

Thus, in addition to providing a lighter weight and more environmentallyfriendly system, according to one embodiment, a more efficient coolingsystem is provided.

Although the present invention and its advantages have been described indetail, it should be understood that various changes, substitutions, andalterations can be made therein without departing from the spirit andscope of the invention as defined by the appended claims.

1. A method for cooling heat-generating structure disposed in an environment having an ambient pressure, comprising the acts of: providing a fluid refrigerant; reducing a pressure of the refrigerant to a first sub-ambient pressure at which the refrigerant has a boiling temperature less than a temperature of the heat-generating structure; bringing the refrigerant at the first sub-ambient pressure into thermal communication with the heat-generating structure, so that the refrigerant boils and vaporizes to thereby absorb heat from the heat-generating structure; and increasing a pressure of the vaporized refrigerant above the first sub-ambient pressure to a second sub-ambient pressure.
 2. The method of claim 1, and further comprising the act of selecting for use as the refrigerant one of water and a mixture of water and ethylene glycol.
 3. The method of claim 1, and further comprising the act of circulating the refrigerant through a flow loop while maintaining the pressure of the refrigerant within a range having an upper bound less than the ambient pressure.
 4. The method of claim 3, and further comprising the act of configuring the loop to include a heat exchanger for removing heat from the refrigerant so as to condense the refrigerant to a liquid.
 5. The method of claim 4, and further comprising the act of causing the heat exchanger to transfer heat from the refrigerant to a further medium having an ambient temperature that is less than the boiling temperature of the refrigerant at the second sub-ambient pressure.
 6. The method of claim 5, and further comprising the act of selecting for use as the medium one of ambient air and ambient water.
 7. The method of claim 3, and further comprising the act of configuring the loop to include a first pump for circulating the refrigerant through the loop.
 8. The method of claim 4, and further comprising the act of removing any air accumulated in the heat exchanger from any leak in the loop.
 9. The method of claim 8, wherein the act of removing any air comprises condensing at least some vapor to liquid.
 10. The method of claim 8, where the act of removing any air comprises raising a pressure of the air above the ambient pressure and venting the air at the raised pressure to the environment.
 11. The method of claim 3, and further comprising accumulating any liquid not evaporated by the heat-generating structure and within the loop.
 12. The method of claim 11, and further comprising pumping the accumulated liquid back into the loop for further thermal communication with the heat-generating structure.
 13. The method of claim 1, including the act of configuring the heat-generating structure to include a plurality of sections that each generate heat; and wherein the act the of bringing the cooling into thermal communication with the heat-generating structure includes the act of bringing respective portions of the coolant into thermal communication with the respective sections of the heat-generating structure.
 14. The method of claim 13, and further comprising the acts of: providing a plurality of orifices; and causing each of the portions of the refrigerant to pass through the respective orifice before being brought into thermal communication with the respective section of the heat-generating structure.
 15. The method of claim 14, and further comprising the act of configuring the orifices to have respective different sizes or to cause the portions of the refrigerant to have respective different volumetric flow rates.
 16. An apparatus, comprising a heat-generating structure disposed in an environment having an ambient pressure, and a refrigeration system for removing heat from the heat-generating structure, the refrigeration system including: a fluid refrigerant maintained at a pressure less than the ambient pressure at which the refrigerant has a boiling temperature less than a temperature of the heat-generating structure; a structure that directs a flow of the refrigerant in the form of a liquid at the sub-ambient pressure in a manner causing the liquid refrigerant to be brought into thermal communication with the heat-generating structure, the heat from the heat-generating structure causing the liquid refrigerant to boil and vaporize so that the refrigerant absorbs heat from the heat-generating structure as the refrigerant changes state; and a compressor that increases the pressure of the vaporized refrigerant to a second sub-ambient pressure.
 17. The apparatus of claim 16, wherein the refrigerant is one of water and a mixture and ethylene glycol.
 18. The apparatus of claim 16, wherein the structure that directs a flow of the refrigerant is configured to circulate the refrigerant through a flow loop while maintaining the pressure of the refrigerant within a range having an upper bound less than the ambient pressure.
 19. The apparatus of claim 18, further comprising a heat exchanger for removing heat from the refrigerant flowing through the loop at the second sub-ambient pressure so as to condense the refrigerant to a liquid.
 20. The apparatus of claim 19, wherein the heat exchanger transfers heat from the refrigerant at the second sub-ambient pressure to a further medium having an ambient temperature greater than a temperature of the heat-generating structure.
 21. The apparatus of claim 20, wherein the medium is one of ambient air and ambient water.
 22. The apparatus of claim 19, wherein the structure that circulates the coolant through the loop includes a pump that effects the circulation of the coolant.
 23. The apparatus of claim 16, and further comprising an accumulator for accumulating liquid that is not condensed by the heat-generating structure.
 24. The apparatus of claim 23, and further comprising a second pump for returning the accumulated liquid to the heat-generating structure.
 25. The apparatus of claim 19, and further comprising an air removal structure attached to the heat exchanger for removing any air trapped in the heat exchanger from leaks in the refrigeration structure.
 26. The apparatus of claim 25, wherein the air removal structure comprises: a second compressor to raise the pressure of the air; a second heat exchanger to remove heat from the air; a separator for separating the air into liquid and gas components; and a vent operable to vent the gas component to the environment.
 27. The apparatus of claim 19, wherein the heat exchanger exchanges heat with an environment having a temperature greater than a temperature of the heat-generating structure.
 28. The apparatus of claim 16, wherein the heat-generating structure includes a plurality of sections that each generate heat; and wherein the structure for directing the flow of the refrigerant brings respective portions of the refrigerant into thermal communication with the respective sections of the heat-generating structure.
 29. The apparatus of claim 16, wherein the structure for directing the flow of the fluid includes a plurality of orifices and causes each of the portions of the refrigerant to pass through the respective orifice before being brought into thermal communication with the respective section of the heat-generating structure.
 30. The apparatus of claim 29, wherein the orifices have respective different sizes in order to cause the portions of the refrigerant to have respective different volumetric flow rates. 